Mock Javascript AWS.RDS.Signer

Question

I have Connection class that is used to connect to AWS Rds Proxy via IAM Authentication. Part of that process is to create a token. I have a function to create the token but now I having a hard time to mock and test it.

Here is the Connection class with setToken method:

class Connection {
    constructor(username, endpoint, database) {
        this.username = username;
        this.endpoint = endpoint;
        this.database = database;
    }

    setToken () {
        let signer = new AWS.RDS.Signer({
            region: 'us-east-1', // example: us-east-2
            hostname: this.endpoint,
            port: 3306,
            username: this.username
        });

        this.token = signer.getAuthToken({
            username: this.username
        });
    }
}

And here I am trying to mock the return value of AWS.RDS.Signer.getAuthToken()

test('Test Connection setToken', async () => {
    AWSMock.setSDKInstance(AWS);
    AWSMock.mock('RDS.Signer', 'getAuthToken', 'mock-token');


    let conn = new connections.Connection(
        'testUser',
        'testEndpoint',
        'testDb');

    conn.setToken();

    console.log(conn.token);
});

I expected to see "mock-token" as the value for conn.token, but what I get is this:

{
  promise: [Function],
  createReadStream: [Function: createReadStream],
  on: [Function: on],
  send: [Function: send]
}

How can I get AWS.RDS.Signer.getAuthToken() to return a mock token?

---

Edit after trying solution from @ggordon

I have tried to get this to work by injecting AWS into the constructor, but still seem to be having the same issue. I think part of my problem is that AWS.RDS.Signer does not support promises, but I'm not entirely sure.

Here is my new code:

The Token class which generates the token. import AWS from 'aws-sdk';

class Token {
    constructor(awsInstance) {
        this.awsInstance = awsInstance || AWS;
    }

    getToken () {
        const endpoint = 'aurora-proxy.proxy.rds.amazonaws.com';

        const signer = new this.awsInstance.RDS.Signer({
            region: 'my-region',
            hostname: endpoint,
            port: 3306,
            username: 'myUser'
        });

        const token = signer.getAuthToken({
                username: 'svcLambda'
            });

        console.log ("IAM Token obtained\n");
        return token
    }
}

module.exports = { Token };

And the test:

test('Should test getToken from Token', async () => {
    AWSMock.setSDKInstance(AWS);
    AWSMock.mock('RDS.Signer', 'getAuthToken', 'mock-token');

    let tokenObject = new tokens.Token(AWS);
    const token = tokenObject.getToken();

    console.log(token);
    expect(token).toStrictEqual('mock-token');
});

The Token class itself works -- it creates the token and the token can be used to make a successful connection to RDS. However, the unit test fails with the actual token returned (from console.log) being this:

{
  promise: [Function],
  createReadStream: [Function: createReadStream],
  on: [Function: on],
  send: [Function: send]
}

Also here is the package.json as requested by @GSSWain

{
  "name": "mylambda",
  "version": "0.0.1",
  "description": "My description.",
  "repository": {
    "type": "git",
    "url": ""
  },
  "scripts": {
    "lint": "eslint src/**/*.js __tests__/**/*.js",
    "prettier": "prettier --write src/**/*.js __tests__/**/*.js",
    "prettier:ci": "prettier --list-different src/**/*.js  __tests__/**/*.js",
    "test": "cross-env NODE_ENV=test jest",
    "test:coverage": "cross-env CI=true jest --coverage --watchAll=false -u --reporter=default --reporters=jest-junit",
    "build": "npm run build:dev",
    "build:dev": "cross-env NODE_ENV=development webpack --config webpack.config.js"
  },
  "dependencies": {
    "mysql2": "^2.2.5"
  },
  "devDependencies": {
    "@babel/core": "^7.6.4",
    "@babel/preset-env": "^7.6.3",
    "aws-sdk": "^2.552.0",
    "aws-sdk-mock": "^5.1.0",
    "babel-jest": "^24.9.0",
    "babel-loader": "^8.0.6",
    "babel-plugin-transform-es2015-modules-commonjs": "^6.26.2",
    "cross-env": "^6.0.3",
    "eslint": "^6.5.1",
    "eslint-config-prettier": "^6.4.0",
    "eslint-plugin-jest": "^22.19.0",
    "jest": "^24.9.0",
    "jest-junit": "^10.0.0",
    "prettier": "^1.18.2",
    "sinon": "^9.0.3"
  },
  "jest": {
    "verbose": true,
    "transform": {
      "^.+\\.js$": "babel-jest"
    },
    "globals": {
      "NODE_ENV": "test"
    },
    "moduleFileExtensions": [
      "js"
    ],
    "moduleDirectories": [
      "node_modules",
      "src"
    ],
    "coverageThreshold": {
      "global": {
        "statements": 100,
        "branches": 100,
        "functions": 100,
        "lines": 100
      }
    }
  },
  "jest-junit": {
    "outputName": "junit_jest.xml"
  }
}

Answer 1

Problem

The AWS instance/object in your test scope is different from the AWS instance/object being used in your setToken method.

aws-sdk-mock mocks this instance

Due to transpiling, code written in TypeScript or ES6 may not correctly mock because the aws-sdk object created within aws-sdk-mock will not be equal to the object created within the code to test.

Also require will return a new instance.

In essence you are mocking an instance in your test while your actual code is using another instance that has not been mocked.

Possible Solutions

Solution 1

You could modify your code to allow you to optionally inject the desired AWS instances to use eg

import AWS from 'aws-sdk';
class Connection {
    constructor(username, endpoint, database,awsInstance) {
        this.username = username;
        this.endpoint = endpoint;
        this.database = database;
        //if the awsInstance is null  or not provided use the default
        this.awsInstance = awsInstance || AWS;
    }

    setToken () {
        let signer = new this.awsInstance.RDS.Signer({
            region: 'us-east-1', // example: us-east-2
            hostname: this.endpoint,
            port: 3306,
            username: this.username
        });

        this.token = signer.getAuthToken({
            username: this.username
        });
    }
}

your code would not need any modifications, however now you can optionally in your tests

test('Test Connection setToken', async () => {
    AWSMock.setSDKInstance(AWS);
    AWSMock.mock('RDS.Signer', 'getAuthToken', 'mock-token');


    let conn = new connections.Connection(
        'testUser',
        'testEndpoint',
        'testDb',
        AWS //pass mock instance
        );

    conn.setToken();
    let actualToken = (await conn.token.promise());

    console.log(conn.token);
    console.log(actualToken);
});

This is only constructor based injection, you could inject it by doing similar in the setToken method.

You will also notice that in the examples provided by aws-sdk-mock and the example above we've extracted the result from the promise object returned. This is because the mock implementation returns a promise object despite the fact that the aws-sdk especially for the AWS.RDS.Signer.getAuthToken supports synchronous operations. This is a constraint based on the library you are using.

Solution 2

You may want to consider another mocking library if you are interested in synchronous calls which based on the examples shared here would better mimic your code/flow. The other alternative is to consider an asynchronous/promise rewrite of your implementations. I leave this decision to you.

A simple alternative could be:

test('Test Connection setToken', async () => {
    AWS.RDS.Signer = function MockSigner() {
        return {
            getAuthToken: function MockGetAuthToken(){ return 'mock-token'; }
        };
    };


    let conn = new connections.Connection(
        'testUser',
        'testEndpoint',
        'testDb',
        AWS //pass mock instance
        );

    conn.setToken();

    console.log(conn.token);
});

Additional References

I've included a snippet of the method used to mock functions for the aws-sdk-mock retrieved from https://github.com/dwyl/aws-sdk-mock/blob/master/index.js#L118 . You'll see that it creates and returns a request

function mockServiceMethod(service, client, method, replace) {
  services[service].methodMocks[method].stub = sinon.stub(client, method).callsFake(function() {
    const args = Array.prototype.slice.call(arguments);

    let userArgs, userCallback;
    if (typeof args[(args.length || 1) - 1] === 'function') {
      userArgs = args.slice(0, -1);
      userCallback = args[(args.length || 1) - 1];
    } else {
      userArgs = args;
    }
    const havePromises = typeof AWS.Promise === 'function';
    let promise, resolve, reject, storedResult;
    const tryResolveFromStored = function() {
      if (storedResult && promise) {
        if (typeof storedResult.then === 'function') {
          storedResult.then(resolve, reject)
        } else if (storedResult.reject) {
          reject(storedResult.reject);
        } else {
          resolve(storedResult.resolve);
        }
      }
    };
    const callback = function(err, data) {
      if (!storedResult) {
        if (err) {
          storedResult = {reject: err};
        } else {
          storedResult = {resolve: data};
        }
      }
      if (userCallback) {
        userCallback(err, data);
      }
      tryResolveFromStored();
    };
    const request = {
      promise: havePromises ? function() {
        if (!promise) {
          promise = new AWS.Promise(function (resolve_, reject_) {
            resolve = resolve_;
            reject = reject_;
          });
        }
        tryResolveFromStored();
        return promise;
      } : undefined,
      createReadStream: function() {
        if (replace instanceof Readable) {
          return replace;
        } else {
          const stream = new Readable();
          stream._read = function(size) {
            if (typeof replace === 'string' || Buffer.isBuffer(replace)) {
              this.push(replace);
            }
            this.push(null);
          };
          return stream;
        }
      },
      on: function(eventName, callback) {
      },
      send: function(callback) {
      }
    };

    // different locations for the paramValidation property
    const config = (client.config || client.options || _AWS.config);
    if (config.paramValidation) {
      try {
        // different strategies to find method, depending on wether the service is nested/unnested
        const inputRules =
          ((client.api && client.api.operations[method]) || client[method] || {}).input;
        if (inputRules) {
          const params = userArgs[(userArgs.length || 1) - 1];
          new _AWS.ParamValidator((client.config || _AWS.config).paramValidation).validate(inputRules, params);
        }
      } catch (e) {
        callback(e, null);
        return request;
      }
    }

    // If the value of 'replace' is a function we call it with the arguments.
    if (typeof replace === 'function') {
      const result = replace.apply(replace, userArgs.concat([callback]));
      if (storedResult === undefined && result != null &&
          typeof result.then === 'function') {
        storedResult = result
      }
    }
    // Else we call the callback with the value of 'replace'.
    else {
      callback(null, replace);
    }
    return request;
  });
}